1. Field of the Invention
The present invention relates to a liquid crystal display panel and a liquid crystal display apparatus, and in particular, relates to a transmissive/transflective liquid crystal display panel and a transmissive/transflective liquid crystal display apparatus.
2. Description of the Related Art
Currently, liquid crystal display apparatuses are widely used in a display screen of various electronic devices. These liquid crystal display apparatuses are popularly and widely used in electronic devices for various purposes of use, and features of the liquid crystal display apparatuses, including thin-shape, lightness, and low power-consumption, are utilized to the greatest extent.
There is a strong demand, especially in mobile devices exemplified by a mobile phone, for a liquid crystal display apparatus that is thinner in shape, lighter in weight, and lower in power consumption, in view of the circumstance that a user needs to carry a mobile device all the time. As such, techniques have been actively developed for further improvements. This is accompanied by the same strong demand directed to liquid crystal display apparatuses used in the mobile devices. Development of techniques is awaited for a liquid crystal display apparatus that is thinner in shape, lighter in weight, and lower in power consumption.
Conventionally, a liquid crystal display apparatus is often constituted by a combination of a liquid crystal panel including a pair of transparent electrode substrates and a liquid crystal layer, and at least one polarizer. In the case of a transmissive or transflective liquid crystal panel, a pair of polarizers are provided on a face of the respective transparent electrode substrates. On the other hand, in the case of a reflective liquid crystal panel, a polarizer is only provided to a transparent electrode substrate that is provided on an observer side.
The liquid crystal panel adopts a cold-cathode discharge fluorescent lamp, an LED (Light Emitting Diode) or the like as its light source, and the light source is surrounded by a reflector. Light emitted from the light source needs to evenly illuminate a flat face of the liquid crystal panel. Therefore, light from a dot/line light source is changed into two-dimensional luminescence by a light guide. Further, the light source and the light guide are combined with a lens sheet or a diffusion sheet, thereby forming a light unit that has a uniformed in-plane luminance.
The light unit may be provided on a front-face side (observer side) of the liquid crystal panel, which is called frontlight-type, or on a rear-face side of the liquid crystal panel, which is called backlight-type. Currently, both of them are widely used.
However, with the structure in which the liquid crystal panel is combined with such light unit described above, there arises problems in that the thicknesses of a light guide, a lens sheet, and a diffusion sheet, all of which are used in the light unit, not only cause a total thickness of the liquid crystal module to become thicker, but also cause the weight of the liquid crystal module to become heavier. For this reason, it is extremely difficult with the liquid crystal display apparatus constituted by the combination of the liquid crystal panel and the conventional light unit to satisfy the tough demand for a thinner and lighter mobile device.
In view of solving the problems, there are suggested techniques for using a transparent electrode substrate of a liquid crystal panel as a light guide so as to realize a thinner and lighter liquid crystal panel. Such techniques are suggested in, for example, Japanese Unexamined Patent Publications No. 2003-57645 (published on Feb. 26, 2003) and No. 2003-66443 (published on Mar. 5, 2003). Publication No. 2003-57645 teaches a technique (frontlight-type) for using, as a light guide, a transparent electrode substrate that is provided on a front-face side. Publication No. 2003-66443 teaches a technique (backlight-type) for using, as a light guide, a transparent electrode substrate that is provided on a rear-face side. The following describes these conventional techniques.
First, the following describes the technique of frontlight-type taught in publication No. 2003-57645, with reference to FIG. 6.
In the frontlight-type liquid crystal display apparatus of publication No. 2003-57645, the liquid crystal panel is structured in such a way that the liquid crystal layer 103 is provided in a space between a pair of transparent electrode substrates 101 and 102 that sandwich the liquid crystal layer 103, as illustrated in FIG. 6. The transparent electrode substrate 102, which is one of the pair of transparent electrode substrates 101 and 102 that is provided on the observer side, includes a dot/line light source 104 on its end part on a side face. The dot/line light source 104 is realized by an LED, a cold-cathode fluorescent lamp or the like. Further, a polarizer 105 is provided on an outer face of each of the transparent electrode substrates 101 and 102 in the liquid crystal panel.
Further, an optical-path changing layer 106 including protrusions and depressions is provided on a face of the liquid crystal panel, which face is on the observer side. Further, a specular reflection film 107 is provided on a face of the liquid crystal panel, which face is on the rear side.
With the above structure, the function of the light guide is concentrated to the transparent electrode substrate 102 in the liquid crystal display apparatus of publication No. 2003-57645. This makes it possible to reduce the number of components, and thus achieve a thinner shape and a lighter weight.
The following describes the technique of backlight-type taught in publication No. 2003-66443, with reference to FIG. 7.
In the backlight-type liquid crystal display apparatus of publication No. 2003-66443, the liquid crystal panel is structured in such a way that the liquid crystal layer 103 is provided in a space between a pair of transparent electrode substrates 101 and 102 that sandwich the liquid crystal panel, as illustrated in FIG. 7. The transparent electrode substrate 101, which is one of the pair of transparent electrode substrates 101 and 102 that is provided on a rear-face side, includes a dot/line light source 104 at an end part of a side face of the transparent electrode substrate 101. The dot/line light source 104 is realized by an LED, a cold-cathode fluorescent lamp or the like. Further, a polarizer 105 is provided on an outer face of each of the transparent electrode substrates 101 and 102, in the liquid crystal panel.
Further, a low refractive-index layer 116 is provided on a front-face side of the transparent electrode substrate 101 in the liquid crystal panel in such a way that the low refractive-index layer 116 is in contact with the transparent electrode substrate 101. The low refractive-index layer 116 is a layer having a lower refractive index than the refractive index of the transparent electrode substrate 101. Further, a polarizer 117, an optical-path changing layer 117, which includes protrusions and depressions, and a total-reflection film 118 are provided on a rear-face side of the transparent electrode substrate 101. By this way, the transflective liquid crystal display apparatus is realized.
In the above structure, the function of the light guide is concentrated to the transparent electrode substrate 101 in the liquid crystal display apparatus of publication No. 2003-66443. This makes it possible to reduce the number of components, and thus achieve a thinner shape and a lighter weight.
However, the conventional frontlight-type structure of publication No. 2003-57645, in which the transparent electrode substrate provided on the observer side is used as the light guide, has a problem that contrast of a displayed image is degraded. On the other hand, the backlight-type structure of publication No. 2003-66443, in which the transparent electrode substrate provided on the rear-face side is used as the light guide, has a problem that it is not possible to efficiently utilize light emitted from the light source provided on the side face of the transparent electrode substrate, and therefore a bright image cannot be obtained. The following specifically describes these problems.
In the frontlight-type structure of publication No. 2003-57645, the light emitted from the light source 104 provided on the side face of the transparent electrode substrate 102 provided on the front-face side is propagated through the inner part of the transparent electrode substrate 102. Then, the light transmits through the polarizer 105, is reflected totally by an optical-path changing layer 106, which is provided on an upper layer of the polarizer 105 and includes protrusions and depressions. Thereafter, the light enters again the panel, toward the inner part thereof. After reflected by the optical-path changing layer 106, the light transmits through the polarizer 1, then transmits through the transparent electrode substrate 102, and then enters the liquid crystal layer 103. After having transmitted through the liquid crystal layer 103, the light transmits through the transparent electrode substrate 101 provided on the rear-face side. Then, the light is reflected by a specular reflection film 107, which is provided on a back face of the polarizer 105, and then exits toward the observer side.
The path described above is indicated by a path (A) in FIG. 6. Light passing along the path (A) is controlled when exiting the liquid crystal layer 103. By this way, a desired image is displayed.
In the structure of FIG. 6, however, the transparent electrode substrate 102 includes a layer that has a refractive index of approximately 1.5, such as a glass or an alignment layer, and a transparent electrode that has a relatively high refractive index, such as a transparent electrode made of ITO (Indium Tin Oxide). In other words, the transparent electrode substrate 102 has a boundary surface of a multi-layer film where there is a relatively great difference in the refractive indexes. For this reason, after light is emitted from the light source 104, transmitted through the polarizer 105 and the optical-path changing layer 106, and enters again the transparent electrode substrate 102, there may be light that is reflected by any boundary surface of the multi-layer film of the transparent electrode substrate 102 before entering the liquid crystal layer 103, and transmits through the transparent electrode substrate 102 and then through the polarizer 105, as indicated by a path (B) in FIG. 6.
This light is not subject to the control conducted at the liquid crystal layer 103, and therefore becomes excess light leakage. This causes a decrease in the contrast of the displayed image.
On the other hand, in the backlight-type structure of publication No. 2003-66443, the light source 104 is provided on a side face of the transparent electrode substrate 101 provided on the rear-face side. In this structure, after emitted from the light source 104, light is propagated through the inner parts of the transparent electrode substrate 101 and the polarizer 105. Then, the light enters an optical-path changing layer 117, which is provided behind the transparent electrode substrate 101, and then is reflected by a reflection film 118, as indicated by a path (A) in FIG. 7. The light thus reflected by the reflection film 118 is controlled at the liquid crystal layer 103, and then transmits through the transparent electrode substrate 102 and the polarizer 105 that are provided on the observer side. By this way, an image is displayed.
In the structure of FIG. 7, if light directly enters a low refractive-index layer 116 after emitted from the light source 104, the light is reflected totally by the low refractive-index layer 116, as indicated by a path (B) in FIG. 7. As such, the contrast would not be decreased due to excess light leakage.
However, in the case where the light emitted from the light source 104 directly enters the low refractive-index layer 116 and then is reflected totally by the low refractive-index layer 116, the light often exits from a side face of the transparent electrode substrate 101, which side face is opposite to the side face on which the light source 104 is provided. If the light exits from the opposite side face of the transparent electrode substrate 101, the light obviously does not play a role as display light for a displayed image. Accordingly, with the structure of FIG. 7, the loss of light that is emitted from the light source 104 increases, and the light emitted from the light source 104 cannot be utilized efficiently. Thus, it is not possible to display a bright image.